Forces and Balanced Forces

Select a course.

arrow_back

infoWhy this? Forces explain changes in motion and shape, turning effects, work done, pressure, and floating. Representing forces with diagrams and resultants, and investigating springs and fluids, allows us to predict how interacting objects and materials will behave.

scheduleWhy now? This unit formalises primary ideas about gravity, friction, air resistance, levers, and pulleys through force diagrams, calculations, and practical investigation. It prepares us to apply gravity in Space Physics and to develop a quantitative account of speed, acceleration, and resultant force in Year 8 Forces and Motion.

A force is a push or pull that can change the motion or shape of an object.
An interaction pair (Newton's Third Law pair) consists of two forces that are equal in magnitude and opposite in direction, acting on two different objects.
Contact forces act when objects are physically touching, such as friction, tension, and normal contact force.
Non-contact forces act at a distance, such as gravity, magnetic force, and electrostatic force.
The resultant force is the single force that has the same effect as all the forces acting on an object combined.
A zero resultant force means that an object remains stationary or continues at constant velocity, whereas a non-zero resultant force causes a change in velocity.
A moment is the turning effect of a force about a pivot, calculated as force multiplied by the perpendicular distance from the pivot.
Resistant forces are forces that oppose motion, such as friction, air resistance (drag), and water resistance.
Friction and drag can be reduced by streamlining objects, using lubricants, or using smoother surfaces.
Objects can be deformed by stretching, compressing, bending, twisting, or shearing.
A direct proportionality is shown by a straight-line graph through the origin; for a spring obeying Hooke's law, force is directly proportional to extension up to the limit of proportionality.
Work done is the energy transferred when a force moves an object over a distance, calculated as work done equals force multiplied by distance moved in the direction of the force.
Pressure is defined as force per unit area, calculated as pressure equals force divided by area.
Atmospheric pressure is the pressure exerted by the weight of the air above a surface.
Atmospheric pressure decreases with increasing height above sea level.
Liquid pressure increases with depth due to the weight of the liquid above.
Objects float if their average density is less than the density of the fluid they are in; they sink if their average density is greater.

rocket_launchYou must be able to

Draw force diagrams in one dimension, accurately representing the size and direction of forces acting on an object.
Use calculations to determine if forces on an object are balanced (resultant force is zero) or unbalanced (resultant force is not zero).
Calculate the moment of a force using the equation: moment equals force multiplied by perpendicular distance from the pivot.
Add arrows to force diagrams to represent resistant forces such as friction and drag.
Describe and carry out a method to investigate how the extension of a spring changes as different forces are applied, for example using a clamp stand, spring, masses, and a ruler.
Calculate the work done when a spring is stretched using the formula: work done equals 0.5 multiplied by force multiplied by extension (for springs obeying Hooke's Law).
Calculate pressure using the formula: pressure equals force divided by area.
Explain, using knowledge of density and upthrust, why some objects sink and some float.

Curriculum Portal